1. Field of Invention
The present invention relates to an organic light emitting diode (OLED) device. More particularly, the present invention relates to an OLED device having a black layer therein.
2. Description of Related Art
Due to rapid growth in the communication industry, portable equipment is a major center of development. Flat displays have become the principal human-machine interface. In general, flat panel displays are divided according to the types of techniques into plasma display panel (PDP), liquid crystal display (LCD), electro-luminescent display, light emitting diode (LED), field emission display (FED) and electro-chromic display. However, organic light emitting diode (OLED) device has the greatest potential to become the dominant flat panel display in the next generation. This is because the OLED display has many advantages including self-illuminating, no viewing angle retention, low energy consumption, easy to fabricate, low production cost, low operating temperature, quick response and full coloration.
FIG. 1 is a cross-sectional view of a conventional organic light emitting diode device. A conventional OLED device mainly includes a transparent substrate 100, a plurality of transparent anode strips 102, an organic functional layer 104 and a plurality of metallic cathode strips 106. The transparent substrate 100 is typically a glass substrate. The transparent anode 102 is made from a transparent conductive material such as an indiumtin oxide. The organic functional layer 104 is a stack of organic films typically includes a hole injection layer, a hole transporting layer, an organic electro-luminescent layer, an electron transporting layer and an electron injection layer. The metallic cathode 106 is usually made from aluminum, calcium or magnesium-silver alloy.
In general, for any types of display device, the ratio between full brightness and full darkness is a major factor determining the quality of the display. This brightness ratio is often referred to as the contrast ratio (CR). A large contrast ratio indicates a bigger contrast between objects. The contrast ratio (CR) is defined by the following formula (1)                     CR        =                                            L                              sub                ,                on                                      +                          R              amb                                                          L                              sub                ,                off                                      +                          R              amb                                                          (        1        )            
where Lsub,on is the brightness level when a pixel is on, Lsub,off is the brightness level when the pixel is off and Ramb is the brightness level when external light enters the display device and gets reflected. If the brightness level is 100 when the pixel is on, and the brightness level is 1 when the pixel is off, the brightness level of the light that enters the display device and then reflects back out can be computed through the contrast ratio formula (1).
FIG. 2 is a graph showing the relationship between the contrast ratio of a conventional organic light emitting diode device and external brightness level. As shown in FIG. 2, the contrast ratio will decrease as brightness level of light from an external source is increased. In other words, image contrast will deteriorate as more light enters the display device. Thus, designing a display device having a high contrast ratio for increasing image clarity is a now major area of inquiry.
The company Luxell has developed an organic light emitting diode device having an optical interference layer. The OLED device design mainly includes inserting a very thin semi-transparent metallic layer and a transparent material layer between an organic light emitting layer and a metallic cathode. The transparent material layer is typically a metallic oxide layer. Through the destructive optical interference effect of the aforementioned structure, reflection is reduced to below 1% of the incoming external light. Hence, contrast ratio of the display device is increased considerably.
However, the organic functional layer inside the OLED is formed by evaporation while the transparent material layer (a metallic oxide layer) is formed by a sputtering. Major drawbacks of the aforementioned methods includes:
1. The fabrication of the organic light emitting layer and the optical interference layer must be carried out in different stations.
2. The fabrication of those thin films in different stations often leads to a stress mismatch between the films resulting in a drop in yield and an increase in production cost.
3. Because sputtering is the bombardment of a target with an energetic beam of ions, the process of forming the optical interference layer may lead to some damages in the organic light emitting layer.
4. Cost of the sputtering equipment (station) necessary for forming the optical interference layer is usually very expensive, thereby jacking up the investment cost.
Accordingly, one object of the present invention is to provide an organic light emitting diode (OLED) device having a black layer that can greatly reduce the amount of back reflection of incoming external light and increase the contrast ratio under strong illumination. Ultimately, image clarity of the display device is enhanced.
A second object of this invention is to provide an organic light emitting diode (OLED) device having a black layer fabricated by evaporation. Hence, the fabrication of the black layer and the organic light emitting layer are compatible with each other leading to a simpler manufacturing step. Moreover, the fabrication of the black layer through evaporation spares the organic light emitting layer from any damages.
A third object of this invention is to provide an organic light emitting diode (OLED) device having a black layer fabricated using a common organic light emitting material. Hence, the process of forming the black layer requires no additional evaporation material or equipment.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an organic light emitting diode (OLED) device. The OLED device mainly comprises of a transparent substrate, a plurality of anodes, an organic functional layer, a black layer and a plurality of cathodes. The anodes are positioned over a transparent substrate. The organic functional layer is positioned over the transparent substrate covering the anodes. The black layer is positioned over the organic functional layer and the cathodes are positioned over the black layer. Through the black layer, the amount of back reflection of external light is reduced and the contrast of the display device is increased.
In this invention, the anodes and the cathodes have a strip-like structure, for example. Moreover, the anodes extend in a direction perpendicular to the cathodes. The black layer is a composite layer that includes an organic material layer and a semi-transparent layer. The organic material layer is made, for example, from an organic compound or an organometallic compound such as copper phthalocyanine (CuPc), which has electron transporting capability. The semi-transparent layer is, for example, a very thin metallic layer such as an aluminum layer.
Furthermore, the organic functional layer in this invention is a composite layer that includes, for example, an hole injection layer, a hole transporting layer, an organic electro-luminescent layer, an electron transporting layer and an electron injection layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.